IVA test fail: Steering turning circle correction

[WillF71]

Hi all.
I have recently had a fail on the IVA turning circle self-correction with my GBS Zero. At the low speed turn, taking my hands off the wheel has no change at all in the turning circle. It simply stays on track with the same turning radius. Any ideas on what I could do with the front geometry to try and initiate self- correct?
I have over-inflated front tires. Steering rack is a new Mk2 escort unit and seems free with the front wheels in the air so I don't think there is any binding.

Question would be, in general, which element of the front geometry has the greater affect on this self-centering?

P.S. It does have an LSD too.

Thanks in advance

Will

[dandan62]

Hi. It's the castor angle that effects the self centering. I'm not familiar with the Zero but I'm sure someone with help you with how to adjust it. Good luck.  

[WillF71]

Thanks for the comments. 

My Mk2 version of the GBS uses Mazda front uprights.  The upper wishbones look real similar to that of a Ford-based kit (only from images I can find) and I can't see any Castor adjustment other than reducing my Camber to absolute minimum and seeing if that helps the steering effect.  Shall try that next weekend and see how it goes.

[agent_zed]

Are there any washers/spacers on the wishbones that you can move from one side to the other of the bushes? so the top wishbone moves backwards and the bottom wishbone moves forward.

[blue]

The easy answer is to ask GBS.  They IVA these all the time- buy the time you mess around changing this and that one phone call will get you a pass.  

[WillF71]

@ Blue, I called them yesterday regarding this, and emissions testing and they were useless.  Suggested there was no Castor adjustment, the MX5 setup that I am using is difficult and that I should just pump up the tires.  Far from happy!!!

[dandan62]

You can't be the first Zero owner with this issue. Have you tried asking on the Gbs zero Facebook group?

[nelmo]

I had the same problem but easily solved - get a laser alignment done on the front wheels and make sure you have them either dead straight or with a slight amount of toe-in.

I just squeezed through IVA and then had the alignment done afterwards. I got a mate to do it with one of those DIY kits he had - I tried to go to a pro but he couldn't get his kit underneath my wheel arches.

[Sparepart]

I have the same behaviour on an Exmo, obviously no use to you to know this, however in the days of SVA it wasn't an issue, the tester made no comment about having to unwind the steering to straighten out from a turn. I now know that most of the problem was related to the really poor bearing set up on top of the strut, causing lots of friction. As mentioned above by agent_z anything you can do to push the bottom wishbone forward and the top one back is going to help. If you have bushes that are pressed into the wishbones then perhaps you can press them off center to get some adjustment. Also I suppose that a lowering of the back of the car would have some effect on the angle of castor at the front, do you have any rear ride height adjustment, or raise the front as well. Oh yes, something else I found was at the universal joint where the column is connected ot the rack. When the weight was off the front wheels I could easily move from lock to lock just by pushing/pulling on the wheels. However when the car was back on the ground, the pressure on that UJ means that the small amount of play in it caused the top of the lock nut to slightly foul on a bit of poor casting on the yoke. Just a few strokes of a file to remove the casting mark and the steering was much free-er.

[Pepsi51]

to check what you have, Set  the chassis on stands as level as you can and remove one of the front hubs so you can see both top and bottom ball joints. get a builders level in upright position aligning with bottom joint. The top joint should be trailing the bottom joint. the trail should be about 10-12 degrees. If its not trailing then there is the problem. you will need to adjust this trail by either washers on the inner bush connections or the body mounts. In some cases the top wishbone might have been fitted wrong way round.

If you have too much turning angle on the rack this can counter the caster effect on full lock. Restricting the turning angle will help.

[Snapperpaul]

A bit of toe out and pump the tyres up to 30psi or more, this is what we did to get a car past SVA and another past IVA then investigated and fixed the issue.